Planet hunters find an Earth-mass planet and a potential water world

One star, its fourth planet, and a lot of buzz.

That's what's happening as planet-hunters digest the news that European astronomers have detected planet No. 4 orbiting a star with the imaginative moniker: Gliese 581. The new planet has 1.9 times Earth's mass, making it the tiniest exoplanet astronomers have bagged to date.

The golden Easter egg in this hunt, of course, is the elusive Earthlike planet orbiting a star in its habitable zone. There, it's not too hot and not too cold, but just right for liquid water to gather and persist on the planet's surface.

To spot a planet this small, and use an Earth-based telescope to do it, is a big deal.

"This is really the most exciting new discovery in the field of exoplanets," enthused Debra Fischer, an astronomer at San Francisco State University and an active planet hunter, in an e-mail exchange.  It's an impressive achievement, she continues, in no small part because the planet's tell-tale signature is tiny compared with larger planets astronomers have found using the same detection technique.

Looking for stars that wobble

The approach detects the presence of a planet through the tiny tug its gravity imparts on its host star. This appears as a periodic wobble in the star's spectrum. The tool in this case was a sensitive planet-hunting spectrograph called HARPS bolted to the back end of the European Southern Observatory's 3.6-meter telescope at La Silla, Chile.

In the process of nabbing this new planet, the European team also refined estimates of the orbits of three other planets in the system – a revision that pulled a planet with seven times Earth's mass more securely into Gliese 581's narrow habitable zone. That planet, Gliese 581d, was discovered in 2007, and orbits the star once every 66.8 days.

Gliese 581d "is probably too massive to be made only of rocky material, but we can speculate that it is an icy planet that has migrated closer to the star," according to Stephane Udry, an astronomer with the University of Geneva's Geneva Observatory and a member of the research team. This would make it "the first serious water-world candidate," she noted in a prepared statement.

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